1. Field of the Invention
The present invention relates to a DC power supply using semiconductor devices. More specifically, the present invention relates to a soft-switching DC power supply including a resonant circuit for reducing switching losses in semiconductor devices.
2. Description of the Related Art
A power supply for converting direct current to direct current (hereinafter referred to as a DC power supply) is used to stabilize unstable direct current voltage, change direct current voltage, or provide electric isolation between input and output. Especially, in a DC power supply for providing electric isolation between input and output, a method of downsizing an isolation transformer by increasing an applied frequency is known.
FIG. 8 shows a circuit configuration disclosed in Japanese Patent Laid-Open Publication No. 2010-178501 (Patent Document 1) as an example. A DC power supply shown in FIG. 8 includes: a DC voltage source 100; a converter 102 for converting DC power outputted from the DC voltage source 100 into AC power; a transformer 103 for inputting the AC power outputted from the converter 102; a rectifier circuit 105 for converting the AC power outputted from the transformer 103 into DC power; a filter reactor 108 and filter capacitor 112 for smoothing the DC power outputted from the rectifier circuit 105; and a load 113 connected in parallel to the filter capacitor 112.
In the DC power supply shown in FIG. 8, switching losses in semiconductor devices composing the converter 102 are increased in accordance with an applied frequency. Accordingly, a technique (soft-switching) for reducing switching losses using a resonant circuit is suggested. FIG. 9 shows a circuit configuration disclosed in Japanese Patent Laid-Open Publication No. 4-368464 (Patent Document 2) and O. Deblecker, A. Moretti, and F. Vallee: “Comparative Analysis of Two zero-Current Switching Isolated DC-DC Converters for Auxiliary Railway Supply” SPEEDAM2008 (Non-Patent Document 1) as an example.
A DC power supply shown in FIG. 9 includes: a DC voltage source 100; a converter 102 for converting DC power outputted from the DC voltage source 100 into AC power; a transformer 103 for inputting the AC power outputted from the converter 102; a rectifier circuit 105 for converting the AC power outputted from the transformer 103 into DC power; a resonant circuit which is composed of a resonant switch 106 and a resonant capacitor 107 and which is connected in parallel to a DC output side of the rectifier circuit 105; a filter reactor 108 and filter capacitor 112 for smoothing the DC power outputted from the rectifier circuit 105; and a load 113 connected in parallel to the filter capacitor 112.
The DC power supply shown in FIG. 9 activates the resonant switch 106 at the timing of turning off the converter 102 to superpose a resonant current Iz on a secondary current I2. Accordingly, the secondary current I2 can be temporarily reduced to zero and a primary current I1 can be temporarily reduced to a level of only excitation current. By turning off the converter 102 at this timing, the turn-off power loss of the converter 102 can be considerably reduced.
In the DC power supply shown in FIG. 8, while semiconductor devices Q1 to Q4 constituting the converter 102 are off, the primary current I1 and the secondary current I2 are zero but a free wheeling current continues to flow through diodes constituting the rectifier circuit 105. When the semiconductor devices Q1 and Q4 constituting the converter 102 are turned on, the primary current I1 and the secondary current I2 start to flow and the magnitude of the secondary current I2 matches that of a load current Id. At this time, a current having the same magnitude as that of the secondary current I2 flows through a half of the diodes constituting the rectifier circuit 105 and no current flows through the other half of the diodes.
A voltage waveform and a current waveform of the latter diodes are shown in FIG. 10. When a state where a current flows through the diodes is shifted to a state where the current is interrupted and a voltage is applied, carriers stored in the diodes are discharged and a current (reverse recovery current) temporarily flows in a reverse direction. Then, a serge voltage is generated. The surge voltage continues to oscillate for a while due to resonance caused by junction capacitance of the diodes and circuit inductance. A recovery phenomenon depends on characteristics of diodes. When the surge voltage is intensely generated as shown in FIG. 10, the surge voltage may exceed a device withstanding voltage and the devices may be destroyed. Even when the surge voltage does not exceed the device withstanding pressure, an electromagnetic noise of a high frequency may be generated, and accordingly, other electronic devices may be negatively affected. For example, they may be electromagnetically interfered.
As a countermeasure against a surge voltage during power recovery, a technique of providing a CR circuit (snubber circuit) in parallel to diodes constituting a rectifier circuit is known. As conventional examples of the countermeasure against the surge voltage, a DC-DC converter disclosed in Japanese Patent Laid-Open Publication No. 2006-352959 (Patent Document 3), a device for transmitting electric power disclosed in Japanese Patent Laid-Open Publication No. 2009-273355 (Patent Document 4), and a low-loss converter disclosed in Japanese Patent Laid-Open Publication No. 2008-79403 (Patent Document 5) are known.
The circuit shown in FIG. 9 as a conventional example has three problems. The first problem is how to reduce the surge voltage of the diodes constituting the rectifier circuit 105 during the power recovery.
The second problem is that it is difficult to recognize a correct load state from the secondary current I2 because the resonant current Iz, which flows through the resonant circuit composed of the resonant capacitor 107 and resonant switch 106 added to reduce the turn-off loss of the converter 102, is superposed on the secondary current I2. A current sensor for directly detecting the load current Id may be provided to recognize the correct load state. The problem here is, however, how to correctly estimate the load current Id from the secondary current I2 for saving the cost and recognize the correct load state without the current sensor for detecting the load current Id.
The third problem is how to perform the smooth control when the load is light. Since the amplitude of the resonant current Iz flowing through the resonant capacitor 107 is proportional to a DC input voltage Vs, an energy supplied by the resonant current Iz is proportional to the square of the DC input voltage Vs. When the DC input voltage Vs is high, a considerably high energy is supplied only by the resonant current Iz. Thus, it is difficult to reduce the supplied electric power.